When designing a wind turbine generator system (WTGS), the designer must choose between different generator concepts such as direct-drive/gearbox and electrical excitation/permanent magnet. A direct-drive generator is generally considered to be more reliable than one with a gearbox since gearboxes tend to have very limited service life in wind turbine applications. Moreover, direct-drive generators operate in variable speed and therefore generate more energy for a given wind speed, they have minimal losses due to transmission of torque from the turbine blades to the generator, and they can easily control active and reactive power. Because of their mechanical reliability, direct-drive generators are especially suitable for far-away wind power plants such as off-shore plants where the need for long service intervals is obvious. In wind turbine applications which are characterized by high torque and low speed the major drawbacks of direct-drive generators are high cost and large size i.e. poor torque density.
In order to improve the torque density of direct-drive wind turbine generators, high magnetic flux density is achieved by using electrical excitation rather than permanent magnets. To further reduce the dimensions of the machine and in order to avoid copper losses, superconducting generators are developed. US2008/0197633 discloses a generator comprising a stationary superconducting field winding surrounded by a rotating armature winding. The field winding comprises a plurality of superconducting coils which are housed in a large cryostat to cool the coils below the transition temperature i.e. the temperature below which the electrical conductor material of the coils becomes superconducting.
One problem with the prior art generators described above is that their electrical components are sensitive for defects. Such defects may occur in the field windings, armature windings, related power electronics, an eventual cryostat and a transformer. If one of these components fails, the whole generator becomes useless and needs to be repaired before an operation of the wind turbine can be resumed. Another problem is that required maintenance actions become excessive when a whole rotor, a stator, one large power converter, one large cryostat or one large transformer needs to be replaced or repaired.
A service-friendly electrical motor is known from EP0909002 which discloses a stator divided into a plurality of magnetic stator units, each unit being independent and removable in axial direction. This motor can be operated even when failed magnetic stator units are disconnected, and the output power can be restricted by actively disconnecting part of the magnetic stator units. The motor disclosed in EP0909002 is of a permanent magnet type and can therefore not experience electrical defects in the excitation part of the motor.